Start update to musl v1.2.3

[xbjfk]

Description

This is effectively a partial continuation of #3068 (no time64 yet), however done behind a flag so people can test changes in 0.2. Some changes are not behind the musl_v1_2_3 config flag, and these are:

• The change to RLIM_NLIMITS. This one is definitely safe, as it has had a deprecation warning saying it could change any time with OS version for >5 years
• The change to O_LARGEFILE is likely safe.
• Same with the change to the tcp_info struct, as these only add fields while keeping ABI. However, if manually initializing all fields this could cause breakage.

Let me know if any of m assumptions are incorrect and I am happy to gate those changes behind the cfg as well.

I've also added #4437, as this seems like a good chance to rename the field while the cfg option is in place.

Sources

O_LARGEFILE change: bminor/musl@b8b729b
RLIM_NLIMITS change: bminor/musl@2507e7f
utmpx change: bminor/musl@1e7f0fc
tcp_info changes: bminor/musl@d4f2981, bminor/musl@5e0c9f2

Checklist

• (rest of) time64 API - this can be done in another PR after this one is merged
• CPU_SETSIZE change (not urgent as changed after 1.2.3)
• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see rust-lang/libc#3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated

[xbjfk]

hmm, looks like I need to allow deprecation for CI to pass - will allow deprecated tomorrow.

[tgross35]

Thank you for working on this! A handful of small comments, most importantly we don't want to stop testing the current musl version because that needs to continue working until we do the final version bump. Also looks like there is a CI failure.

[tgross35]

⚠️ Warning ⚠️

* There are issue links (such as `#123`) in the commit messages of the following commits.
  _Please remove them as they will spam the issue with references to the commit._
  
  * [5f03e36](https://github.com/rust-lang/libc/commit/5f03e36a51907a467127c9a722039842ff1fd066)

Disregard this, that's not a good warning

[tgross35]

This looks pretty good as a first step to me, I think we can merge it once you get CI passing.

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[xbjfk]

Thanks for the review! Looks like the CI passes, except for freebsd (which seems to have had failures previously)

I've started working on the time64 changes, expect a PR for those in a few days if all goes to plan :)

@rustbot ready